1. Field of the Invention
This invention relates generally to a heated valve and, more particularly, to a heated valve used in an anode outlet unit for a fuel cell system, where the valve includes a ceramic ring heater positioned proximate to a valve seat within a valve body.
2. Discussion of the Related Art
Hydrogen is a very attractive fuel because it is clean and can be used to efficiently produce electricity in a fuel cell. A hydrogen fuel cell is an electrochemical device that includes an anode and a cathode with an electrolyte therebetween. The anode receives hydrogen gas and the cathode receives oxygen or air. The hydrogen gas is dissociated at the anode catalyst to generate free protons and electrons. The protons pass through the electrolyte to the cathode. The protons react with the oxygen and the electrons at the cathode catalyst to generate water. The electrons from the anode cannot pass through the electrolyte, and thus are directed through a load to perform work before being sent to the cathode.
Proton exchange membrane fuel cells (PEMFC) are a popular fuel cell for vehicles. The PEMFC generally includes a solid polymer electrolyte proton conducting membrane, such as a perfluorosulfonic acid membrane. The anode and cathode typically include finely divided catalytic particles, usually platinum (Pt), supported on carbon particles and mixed with an ionomer. The catalytic mixture is deposited on opposing sides of the membrane. The combination of the anode catalytic mixture, the cathode catalytic mixture and the membrane define a membrane electrode assembly (MEA). MEAs are relatively expensive to manufacture and require certain conditions for effective operation.
Several fuel cells are typically combined in a fuel cell stack to generate the desired power. For example, a typical fuel cell stack for a vehicle may have two hundred or more stacked fuel cells. The fuel cell stack receives a cathode input gas, typically a flow of air forced through the stack by a compressor. Not all of the oxygen is consumed by the stack and some of the air is output as a cathode exhaust gas that may include water as a stack by-product. The fuel cell stack also receives an anode hydrogen input gas that flows into the anode side of the stack.
A fuel cell stack includes a series of bipolar plates positioned between the several MEAs in the stack, where the bipolar plates and the MEAs are positioned between two end plates. The bipolar plates include an anode side and a cathode side for adjacent fuel cells in the stack. Anode gas flow channels are provided on the anode side of the bipolar plates that allow the anode reactant gas to flow to the respective MEA. Cathode gas flow channels are provided on the cathode side of the bipolar plates that allow the cathode reactant gas to flow to the respective MEA. One end plate includes anode gas flow channels, and the other end plate includes cathode gas flow channels. The bipolar plates and end plates are made of a conductive material, such as stainless steel or a conductive composite. The end plates conduct the electricity generated by the fuel cells out of the stack. The bipolar plates also include flow channels through which a cooling fluid flows.
The MEAs are porous and thus allow nitrogen in the air from the cathode side of the stack to permeate therethrough and collect in the anode side of the stack, referred to in the industry as nitrogen cross-over. Nitrogen in the anode side of the fuel cell stack dilutes the hydrogen concentration such that if the nitrogen concentration increases beyond a certain percentage, such as 50%, the fuel cell stack may become unstable and may fail. It is known in the art to provide a bleed valve at the anode gas output of the fuel cell stack that is periodically opened to remove the nitrogen from the anode side of the stack.
As discussed above, it is necessary to periodically bleed the anode exhaust gas because of nitrogen accumulation in the anode side of the fuel cell stack. However, when the anode exhaust gas is bled, hydrogen is also included in the anode exhaust gas that could present a combustion problem outside of the fuel cell system. Therefore, it is known in the art to combine the anode exhaust gas with the cathode exhaust gas to reduce the concentration of exhausted hydrogen below a combustible level. Control models are known in the art to determine how much hydrogen is in the bled anode exhaust gas. Particularly, these algorithms know the pressure difference across the fuel cell stack and the flow of the anode exhaust gas through the bleed valve orifice, which can be used to determine the concentration of hydrogen. However, if significant water and water vapor exists within the anode exhaust gas, then the flow characteristics of the gas through the bleed valve are not able to be accurately determined. Thus, it is necessary to separate the water and water vapor from the anode exhaust gas before it is bled through the bleed valve.